1. Field of the Invention
The present invention is generally related to the field of semiconductor processing, and, more particularly, to a method of controlling positioning of a focal plane of an exposure tool relative to a surface of a semiconductor.
2. Description of the Related Art
In general, semiconductor devices are manufactured by forming many process layers comprised of various materials above a semiconducting substrate, and, thereafter, removing selected portions of the layers, i.e., patterning the layers. This patterning may be accomplished using known photolithography and etching processes to define the various features of the device, e.g., the gate insulation layer, the gate electrode, sidewall spacers, metal lines and contacts, etc. This forming and patterning of the process layers is typically performed layer by layer as the individual layers are formed, although multiple layers may be patterned at any given time.
Photolithography is a common process used in patterning these various layers. Photolithography typically involves the use of a product known as photoresist. In general terms, photoresist is a product that may be changed from a relatively soluble state to a relatively insoluble state by exposure to a light source. There are positive and negative photoresist currently available on the market.
In general, the photolithography process involves forming a layer of photoresist above a previously formed process layer, and exposing selected portions of the layer of photoresist to a light source to form a pattern in the photoresist that is desired to be formed in the under-lying process layer. All of these steps are typically performed in well-known photolithography modules that include a section for depositing the photoresist on the wafer, e.g., a spin-coating station, a device for selectively exposing portions of the photoresist layer to a light source through a reticle, e.g., a stepper, and a section for rinsing and developing the photoresist layer after it has been selectively exposed to the light source. Thereafter, an etching process, such as a plasma etching process, is performed to remove portions of the underlying process layer that are not covered by the patterned layer of photoresist, i.e., the patterned layer of photoresist acts as a mask. After the etching process is complete, the patterned photoresist layer is removed so that additional process layers may be formed above the now patterned process layer.
The purpose of the photoresist application step is to form a thin, uniform, defect-free film of photoresist above the substrate surface. A typical layer of photoresist may have a thickness varying from approximately 1500-15,000 xc3x85, and it usually is required to have a uniformity of xc2x1100 xc3x85. Even this permissible variation may have undesirable effects. That is, when the photoresist is exposed to the light source, the light is generally focused at a preselected location above the substrate, which during ideal operating conditions coincides with the top surface of the layer of photoresist. However, where the photoresist is thicker or thinner than desired or anticipated, then the focal plane of the light source falls below or above the surface of the photoresist, respectively. Thus, the patterns or features formed in the layer of photoresist may not be as precise as would otherwise occur
Moreover, variations in the thickness of process layers underyling the layer of photoresist may also cause the top surface of the photresist to not coincide with the focal plane of the light source. Thus, the light source may be slightly out of focus, which reduces the precision with which the pattern may be formed in the photoresist. Reduced precision in forming the pattern in the photoresist may produce semiconductor devices that have a variety of undesirable characteristics, such as increased leakage currents, increased capacitance, or in the worst case may render the device inoperable.
The present invention is directed to a method of solving or at least reducing some or all of the aforementioned problems.
In one illustrative embodiment, the present invention is directed to a method comprised of forming a layer of photoresist above a process layer formed above a semiconducting substrate. Thereafter, a position of a top surface of the layer of photoresist is determined, and a focal plane of a light source is positioned adjacent the determined position of the top surface of the layer of photoresist. Subsequently, the light source is energized.
In another embodiment of the instant invention, a system comprises a metrology tool, a controller, and a stepper. The metrology tool senses a thickness of a first layer of photoresist formed above a first semiconducting substrate. The controller determines a position of a top surface of the layer of photoresist based upon the sensed thickness of the layer of photoresist, and is capable of delivering a control signal indicating the position of the top surface of the layer of photoresist. The stepper is capable of moving one of a light source and the substrate to position a focal plane of the light source at about the determined position of the top surface of the layer of photoresist in response to receiving the control signal.